1. Field of the Invention
This invention relates to a method for preventing or retarding the microbial or calcareous fouling of a conducting or semiconducting surface in an aqueous environment. More specifically, such a method comprises applying a cathodic voltge and current to a conducting or semiconducting surface to produce at or near the surface an effective concentration of hydrogen peroxide by the in situ reduction of dissolved oxygen.
2. Background
The formation of microbial or calcareous deposits on metallic surfaces in contact with natural waters is a well recognized and long standing problem of universal concern. The formation of such deposits sets the initial stage for more serious biofouling when the ambient water contains bacteria, algae, barnacles, mussels or other organisms. In heat exchange systems, this microbial and calcareous fouling is responsible for the loss of heat transfer capacity, the reduction of water flow and control through fouled lines and the increased corrosion of the heat exchange surfaces.
Previously both mechanical and chemical means have been used in attempts to prevent or retard such fouling. Mechanical cleaning requires that the fouled equipment be shut down and subjected to ultrasonic vibrations, and washed or mechanically scrapped, e.g. by inserting rubber balls or other mechanical devices. Such mechanical cleaning results in significant lost operating time for the equipment. Chemical treatment has used such toxic materials as chlorine and other chlorine based compounds, ozone, organosulfurs and other selected organic toxics. These toxic materials are generally added to the bulk water supply. Such bulk addition of large quantities of toxic materials may be both expensive and environmentally unacceptable by producing large quantities of toxic water.
In situ electrochemical techniques have been employed to generate toxic materials. Such techniques have included the anodic generation of chlorine from dissolved chloride ions in salt water environments. The cathodic evolution of hydrogen at relatively high current densities, e.g., 1-10 milliamperes per square centimeter, was partially successful in retarding microbial fouling but resulted in increased calcareous deposits because of the highly alkaline (approximate pH 11.5-13.0) condition created near the surface. The anodic electrochemical generation of oxygen and the production of an acidic condition near the anode surface has been suggested as a means of achieving biofouling and scale control. Current densities greater than 0.1 milliampere per square centimeter are required. A disadvantage of this process is that the anode must be constructed of or coated with a special corrosion resistant and electrocatalytic coating, such as titanium, certain noble metal alloys or certain mixtures containing at least one noble metal oxide.
Such prior methods of preventing or retarding biofouling and scale growth are inefficient, expensive and environmentally unacceptable. Mechanical scraping requires the fouled equipment to be out of operation for a significant time, resulting in heavy financial costs. Treatment of the bulk water requires excessive quantities of toxic chemicals and produces environmentally unacceptable quantities of water contaminated with these toxic chemicals or their residues.
The prior attempts to control such fouling by in situ electrochemical methods have required high current densities resulting in significant power costs and have not been completely successful.